Method and system for concurrent computer transaction processing

ABSTRACT

A computer system and method concurrently process transactions directed to computer devices coupled to a bus agent. The method transmits first and second transaction requests from one or more computer processors across a computer bus to the bus agent. The bus agent transmits the first transaction request to a first computer device coupled to the bus agent. In addition, the bus agent transmits the second transaction request to a second computer device before the bus agent has received a transaction response to the first transaction request from the first computer device, thereby concurrently processing the transaction requests. The bus agent includes plural device managers each uniquely associated with one of the computer devices. Each device manager employs a queue pointer into a transaction queue to track each transaction involving the computer device associated with the device. manager.

This application is a divisional of U.S. patent application Ser. No.08/725,576, filed Oct. 3, 1996 U.S. Pat. No. 5,878,235.

TECHNICAL FIELD

The present invention relates to computer transaction processing, andmore particularly, to concurrently processing computer transactions.

BACKGROUND OF THE INVENTION

A computer system includes a set of interconnected components or modulesof three basic types: central processing unit (CPU), memory, andinput/output (I/O). The modules of the computer system are connectedtogether by communication pathways known as busses. A bus is a sharedtransmission medium in that plural computer modules can transmit acrossthe same bus. However, if two modules transmit during the same timeperiod, their signals will overlap and become garbled. Therefore, it isimportant to ensure that only one module transmits across the bus duringa given time period.

The central processing unit or processor communicates with the memoryand I/O modules of the computer system using information transfers knownas transactions. Each transaction includes a transaction request inwhich one of the computer system modules, such as the computerprocessor, requests the transfer of information to or from another oneof the modules. For example, the computer processor could issue atransaction request that writes data to or reads data from the memorymodule. Each transaction also includes a transaction response thatresponds to the transaction request or indicates that an error hasoccurred.

Many computer processors, such as the Intel Pentium Pro processor, issuetransaction requests in a pipelined manner in which plural transactionrequests are outstanding simultaneously. For example, the Pentium Proprocessor bus allows up to eight transactions to be outstandingsimultaneously. With such a pipelined system, care must be taken toensure that the transaction requests are responded to in the same orderas they were issued.

Shown in FIG. 1 is a prior art computer system 10 that includes aprocessor bus 12. The computer system 10 includes a multiprocessorsystem architecture in which first and second computer processors 14, 16are each coupled to the processor bus 12. Coupled to the first computerprocessor 14 is an input device 18, such as a keyboard, mouse, ormicrophone, that inputs information to the first computer processor froma user. The computer system 10 also includes first and second bus agents20, 22 coupled to the first and second computer processors 14, 16 viathe processor bus 12. Coupled to the first bus agent 20 is a PeripheralComponent Interconnect (PCI) bus 24 which is coupled via a videocontroller 26 to a video monitor 28 that outputs information to theuser. An Industry Standard Architecture (ISA) bus 30 also is coupled tothe PCI bus 24 via a PCI/ISA bridge 32. Coupled to the ISA bus 30 are ahard drive 34 and a fax/modem 36. Coupled to the second bus agent 22 isa main memory 38.

Whenever either of the first and second computer processors 14, 16desires to transmit information to or receive information from one ofthe computer devices 26, 34-38 the computer processor issues atransaction request as discussed above. In prior art computer systems,such as the computer system 10 shown in FIG. 1, the first and secondprocessors 14, 16 and the first and second bus agents 20, 22 includetransaction queues 40, 42, 44, 46, respectively, that each keep track ofthe outstanding transactions of the computer system 10. For example, ifthe first processor 14 issues a transaction request to read data fromthe hard drive 34, the first and second processors 14, 16 and the firstand second bus agents 20, 22 insert an indication of the transactionrequest in their respective transaction queues 40-46. Each of the firstand second processors 14, 16 and the first and second bus agents 20, 22includes logic that prevents the transaction requests from beingresponded to out of the sequential order in which the transactionrequests are inserted in the respective transaction queues 40-46. Thus,the transaction queues 40-46 ensure that the various transactionrequests transmitted by the processors 14, 16 and bus agents 20, 22 areanswered with transaction responses in the same order as the transactionrequests were issued.

Although the transaction queues 40-46 enable each of the first andsecond processors 14, 16 and the bus agents 20, 22 to process a singletransaction concurrently, none of the processors 14, 16 or bus agents20, 22 can process plural transaction requests concurrently. Forexample, if the first processor 14 issues a transaction request to readdata from the hard drive 34 and the second processor 16 subsequentlyissues a transaction request to write data to the video controller 26,the two transactions cannot be processed concurrently by the videocontroller 26 and the hard drive 34. Instead, the first bus agent 20will wait until after the hard drive 34 has issued a transactionresponse in response to the first transaction request and the bus agenthas forwarded the transaction response of the processor bus 12 to thefirst processor 14 before transmitting the second transaction request tothe video controller 26. The first bus agent 20 will wait to send thesecond transaction request to the video controller 26 even if the videocontroller 26 is so much faster than the hard drive 34 that the videocontroller could respond to the second transaction request before thehard drive 34 could respond to the first transaction request.

If the first bus agent 20 transmitted the second transaction requestbefore receiving the response to the first transaction request, then thefirst bus agent could transmit the second transaction response on theprocessor bus before transmitting the first transaction response.Transmitting the transaction responses out of order like that wouldcause the first processor 14 to incorrectly receive the secondtransaction response in response to the first transaction request. Assuch, the first processor 14 would be expecting data read from the harddrive 34 and instead would receive the second transaction response fromthe video controller indicating that data was correctly written to thevideo controller. Such transposing of transaction responses couldquickly disable the computer system 10.

SUMMARY OF THE INVENTION

The present invention is directed to a method, system, and bus agent forconcurrent transaction processing. The method transmits first and secondtransaction requests from one or more computer processors across acomputer bus to the bus agent. The bus agent transmits the firsttransaction request to a first computer device. In addition, the busagent transmits the second transaction request to a second computerdevice before the bus agent has received a transaction response to thefirst transaction request from the first computer device, therebyconcurrently processing the transaction requests. The bus agent receivesfirst and second transaction responses on the first and second computerdevices respectively and transmits the first and second transactionresponses to the one or more computer processors. Concurrentlyprocessing the plural transaction requests enables the bus agent toreceive and transmit transaction responses to the one or more computerprocessors sooner than would be possible with the non-concurrenttransaction processing of the prior art.

In one embodiment, the bus agent receives the second transactionresponse after receiving the first transaction response, but the busagent transmits the first transaction response on the computer busbefore transmitting the second transaction response on the computer bus.In another embodiment, the method concurrently processes two transactionrequests directed to the same computer device. In particular, the methodtransmits a third transaction request to the first computer device fromthe bus agent before transmitting the first transaction response to theone or more computer processors.

The preferred embodiment of the bus agent includes a transaction queuethat stores indications of a plurality of transaction requests receivedby the bus agent from one or more computer processors via a computerbus. Also, the bus agent includes a request controller that inserts intothe transaction queue the indications of the transaction requestsreceived by the bus agent. A response controller uses the transactionqueue to ensure that transaction responses responsive to the transactionrequests are transmitted via the computer bus to the one or morecomputer processors according to the sequential order in which thetransaction requests were received. In addition, the bus agent includesa plurality of device managers each associated with a respectivecomputer device coupled to the bus agent. Each device manager isstructured to transmit one of the transaction requests to the computerdevice associated with the device manager. In addition, each devicemanager receives a transaction response in response to the transactionrequest and signals the response controller that a transaction responseis ready to be transmitted to the one or more computer processors. Eachdevice manager stores its transaction response in the transaction queuein a transaction response field associated with the transaction requestto which the transaction response is responsive. The response controllerresponds to the signal from the device manager by transmitting thetransaction response on the processor bus.

A preferred embodiment of the computer system includes a computerprocessor that issues a plurality of transaction requests according to afirst sequential order. The computer system also includes a plurality ofcomputer devices for responding to the transaction requests from thecomputer processor. Further, the computer system includes a bus agentcoupled between the plurality of computer devices and a computer buscoupled to the computer processor. The bus agent includes a plurality ofdevice managers that keeps track of the transaction requests receivedfrom the computer processor via the computer bus. Each of the devicemanagers is associated with a respective one of plurality of computerdevices. Also, each device manager is structured to transmit arespective one of the transaction requests to the computer deviceassociated with the device manager independently from any of thetransaction requests transmitted by any others of the device managers.Preferably the bus agent also includes a response controller and atransaction queue. Each device manager stores a respective transactionresponse in the transaction queue in a transaction response fieldassociated with the transaction request to which the transactionresponse is responsive. The response controller accesses the transactionresponses in the transaction queue according to the first sequentialorder in which the transaction requests were received.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a prior art computer system.

FIG. 2 is a block diagram of a computer system according to the presentinvention.

FIG. 3 is a timing diagram illustrating how the prior art computersystem shown in FIG. 1 processes transactions non-currently.

FIG. 4 is a timing diagram illustrating how the computer system shown inFIG. 2 processes transaction concurrently.

FIG. 5 is a block diagram of a bus agent employed in the computer systemshown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

A computer system 50 that concurrently processes computer transactionsaccording to one embodiment of the present invention is shown in FIG. 2.Like the prior art computer system 10 shown in FIG. 1, the computersystem 50 includes a multiprocessor system architecture having first andsecond computer processors 52, 54 coupled to a processor bus 56. Coupledto the first computer processor 52 is an input device 58, such as akeyboard, mouse, or microphone, that inputs information to the firstcomputer processor from a user. The computer system also includes a busagent 60 coupled to the first and second computer processors 52, 54 viathe processor bus 56. Coupled to the bus agent 60 is a hard drive 62, aPeripheral Component Interconnect (PCI) bus 64, and a main memory 66.The PCI bus 64 is coupled via a video controller 68 to a video monitor70 that outputs information to the user. An Industry StandardArchitecture (ISA) bus 72 also is coupled to the PCI bus 64 via aPCI/ISA bridge 74. Coupled to the ISA bus 30 are a fax/modem 76 and aprinter 78.

The computer processors 52, 54 and bus agent 60 include transactionqueues 80, 82, 84, respectively, for tracking all of the transactionsthat are transmitted across the processor bus 56. Each of thetransaction queues 80-84 tracks every transaction transmitted on theprocessor bus 56 regardless of which of the computer modules 52, 54, 60are involved in the transaction. For example, if the first processor 52issues a transaction request to read data from the memory 66, thetransaction request is entered into each of the transaction queues 80-84even though the second processor 54 is not involved in the transaction.

The bus agent 60 also includes a transaction controller 86, a devicemanager A 88, a device manager B 90, and a device manager C 92 thattogether process the transactions directed to or from the bus agent 60.The transaction controller 86 and the device managers 88-92 enable thebus agent 60 to concurrently process transactions directed to the harddrive 62, the PCI bus 64, and/or the memory 66. Each of the devicemanagers 88-92 is uniquely associated with one of the computer devices62, 64, 66 coupled to the bus agent 60. In the embodiment shown in FIG.2, the device manager A 88 is associated with the hard drive 62, thedevice manager B 90 is associated with the PCI bus 64, and the devicemanager C 92 is associated with the memory 66. Employing a separatedevice manager for each of the computer devices 62, 64, 66 coupled tothe bus agent enables the transactions directed to any one of thecomputer devices to be processed independently from, and thusconcurrently with, the transactions directed to the other computerdevices coupled to the bus agent 60.

It will be appreciated that the bus agent 60 could include more or fewerdevice managers depending on how many computer devices capable ofprocessing transactions are coupled to the bus agent. For example,another device manager can be directly coupled to the video controller68 so that transaction requests directed to the video controller can behandled separately from and concurrently with transaction requestsdirected to the fax/modem 76 or the printer 78. Moreover, a second busagent with one or more device managers may be employed to concurrentlyprocess transactions directed to one or more devices coupled to thesecond bus agent. Preferably, each computer device coupled to one of thebus agents is uniquely associated with a device manager so thattransactions involving the computer device can be processed concurrentlywith transactions involving other computer devices coupled to one of thebus agents.

A comparison of the transaction processing timing of the prior artsystem shown in FIG. 3 with the transaction processing timing of thepresent invention shown in FIG. 4 shows the processing efficiencyobtained by the present invention. In FIG. 3, either or a combination ofthe first and second processors 14, 16 issues six consecutivetransaction requests labeled 0-5. Assuming that transaction request 0 isdirected to a computer device A, such as the hard drive 34, then thecomputer device A processes the transaction request and transmits atransaction response to the first bus agent 20 which forwards thetransaction response to the appropriate one of the processors 14, 16.After the first bus agent 20 has forwarded to the processor thetransaction response 0 to transaction request 0, then the first busagent can transmit the transaction request 1 to a computer device C,such as the video controller 26. Computer device C processes thetransaction request 1, transmits a transaction response 1 to the busagent, and the bus agent forwards the transaction response 1 to theappropriate processor. Transaction requests 2-5 are handled similarly,that is, the first bus agent 20 must transmit to the appropriate one ofthe processors 14, 16 a response to one transaction request beforetransmitting the next transaction request to the appropriate computerdevice for processing.

The timing diagram of FIG. 4 shows how the present inventionconcurrently processes transactions directed to the computer devices 62,64, 66 coupled to the bus agent 60. In response to receiving the firsttransaction request 0 directed to a computer device A, such as the harddrive 62, the device manager A 88 forwards the transaction request 0 tothe computer device A with which it is associated. While the computerdevice A is processing the transaction request 0, the device manager C92 forwards the transaction request 1 to a computer device C, such asthe memory 66, with which it is associated. Similarly, the devicemanager B 90 forwards to its associated computer device B, such as thePCI bus 64, the transaction request 2 for processing while the computerdevice A continues to process transaction request 0. After devicemanager A 92 receives from the computer device A a transaction response0 in response to the transaction request 0, the transaction controller86 forwards the transaction response 0 to the appropriate one of theprocessors 52, 54. In addition, because the device manager C 88 andcomputer device C were able to process the transaction request 1 whilethe transaction request 0 was being processed, the transactioncontroller 86 can transmit the transaction response 1 immediatelyfollowing the transmittal of the transaction response 0.

The concurrent processing of the present invention as illustrated inFIG. 4 contrasts with the prior art non-concurrent transactionprocessing shown in FIG. 3 in which the first bus agent 20 and thecomputer device C cannot even begin to process the transaction request 1until the transaction response 0 is sent to the appropriate one of theprocessors 14, 16. Moreover, a comparison of FIGS. 3 and 4 shows thatthe present invention can concurrently process and transmit transactionresponses 0-5 in the same amount of time as it takes the prior artsystem to process three transactions and transmit transaction responses0-2. In addition, the present invention maximizes the processing abilityof high speed devices, such as computer device C, by enabling the highspeed devices to process transactions concurrently with relatively slowdevices, such as computer device A, rather than requiring the high speeddevices to wait for the relatively slow devices. Conversely, the presentinvention enables the relatively slow devices to begin processingearlier, and thereby finish earlier, than is possible using prior artnon-concurrent transaction processing systems.

A block diagram of the bus agent 60 is shown in FIG. 5. The transactioncontroller 86 includes a request controller 94 and a response controller96 that keep track of transaction requests and transaction responses,respectively. The transaction queue 82 includes four fields: Atransaction request field 98 that stores each transaction request, astatus field 100 that identifies the status of each transaction, adevice identifier field 102 that identifies the computer device to whichthe transaction request is directed, and a transaction response field104 that stores each transaction response. For example, the transaction0 was directed to computer device A and has already been processed asindicated by the "done" in the status field 100. Similarly, transactions1-4 have each already been processed by the appropriate device managerand computer device as indicated in the status field 100 for thetransactions 1-4.

It should be appreciated that the transaction queue 82 can be employedto track all of the transactions transmitted across the processor bus 56regardless of whether the bus agent 60 is involved in the transactions.Alternatively, the bus agent 60 can include separate transaction queuesto track transactions involving the bus agent 60 separately fromtransactions not involving the bus agent 60.

Each transaction request is entered into the transaction queue 82 by therequest controller 94. The request controller 94 employs a requestpointer 106 to index the transaction request that has most recently beenentered into the transaction queue 82. The request controller 94 alsodecodes each transaction request received by the first bus agent 60 andenters in the device identifier field 102 the identifier of the deviceto which the transaction request is directed. In FIG. 5, the requestcontroller 94 has entered into the transaction queue 82 each of thetransaction requests 0-7, of which transaction request 7 was mostrecently entered as indicated by the request pointer 104. Preferably,the request controller 94 enters into the transaction queue 82 theentire transaction request. Alternatively, the request controller 99could enter only an indication of each transaction request in thetransaction queue 82 and store the actual transaction request in aseparate storage location.

The device managers 88, 90, 92 employ queue pointers 108 and 112,respectively, to track the transactions directed to their associatedcomputer devices 62, 64, 66. In response to the request controller 94inserting a new transaction request in the transaction queue 82, each ofthe device managers 88-92 check the device identifier field 102 todetermine if the new transaction request is being directed to thecomputer device associated with the device manager. For example, thedevice identifier field 102 indicates that the transaction request 5 isdirected to the computer device A (e.g., hard drive 62), so the devicemanager A 88 causes its queue pointer 108 to point to the transactionrequest 5 in the transaction queue 82. Similarly, the device manager B90 causes its queue pointer 110 to point to the transaction request 6 inthe transaction queue 82 because the device identifier field 102indicates that the transaction request 6 is directed to the computerdevice B (e.g, PCI bus 64). Likewise, the device manager C 92 causes itsqueue pointer 112 to point to the transaction request 7 because thedevice identifier field 102 indicates that the transaction request 7 isdirected to the computer device C (e.g., memory 66).

In response to receiving a transaction request from its associateddevice manager, each of the computer devices transmits a transactionresponse to its associated device manager. For example, in response toreceiving transaction request 4 from the device manager A 88, thecomputer device A transmits a transaction response 4 to the devicemanager A. In response to receiving the transaction response 4, thedevice manager A 88 updates the status field 100 for transaction 4 toindicate that the device manager A 88 and the computer device A havefinished processing the transaction request 4.

The response controller 96 sequentially checks the status field 100 ofeach transaction to determine when the transaction has been completelyprocessed by the appropriate device manager and computer device. Upondetermining that a transaction has been processed by the appropriatedevice manager and computer device, the transaction controller transmitsthe transaction response across the processor bus 56 to whichever one ofthe computer processors 52, 54 transmitted the transaction request thatis being responded to by the transaction response. The responsecontroller 96 employs a response pointer 114 to keep track of whichtransaction response is next to be transmitted across the processor bus56. In FIG. 5, the response controller 96 has already transmittedtransaction responses 0-3 across the processor bus 56, and thus, theresponse pointer 114 points to the transaction 4.

The device manager A 88 includes a queue manager 116 that interfaces thedevice manager A with the transaction queue 82. In particular, the queuemanager 116 checks each new transaction request that is entered into thetransaction queue 82 by the request controller 94 to determine whetherthe device identifier field 102 indicates that the transaction requestis directed to the computer device A associated with the device managerA. If the transaction request is directed to the computer device A, thenthe queue manager 116 directs its queue pointer 106 at the transactionrequest and retrieves that transaction request from the transactionqueue 82. In addition, when the device manager A 88 receives atransaction response from the computer device A in response to atransaction request, the queue manager 116 updates the status field 100in the transaction queue to indicate that the device manager A hascompleted its processing of the transaction. The device manager A thenchecks the device identifier field 102 of the transaction queue 82 todetermine whether any additional transaction requests have been directedto computer device A. If so, the device manager A begins processing thenext transaction request directed to computer device A.

The device manager A 88 also includes a transaction I/O module 118 thatinterfaces the device manager A with its associated computer device A.In the computer system 50 shown in FIG. 2, the device manager A 88 isdirectly coupled to the hard drive 62, and thus, the transaction I/Omodule 118 communicates directly with the hard drive. In response toreceiving a transaction request from the transaction queue 82, the queuemanager 116 forwards the transaction request to the transaction I/Omodule 118. The transaction I/O module 118 transmits the transactionrequest to the hard drive 62 for processing. Similarly, when the harddrive 62 responds to the transaction request by creating wandtransmitting a transaction response, the transaction I/O module 118receives that transaction response from the hard drive 62.

In response to receiving a transaction response from the hard drive 62,the transaction I/O module 118 stores the transaction response in the.transaction response field 104 of the transaction queue 82. After thequeue manager 116 has updated the status field 100 of the transaction toindicate that a transaction response has been received, the responsecontroller 96 accesses the transaction response stored in transactionresponse field 104. The response controller 96 retrieves the transactionresponse from the transaction response field 104 and transmits thetransaction response across the processor bus 56 to the appropriate oneof the processors 52, 54. The transaction response field 104 may belimited to storing a single transaction at a time. Alternatively, thetransaction response field 104 can be designed to store pluraltransaction responses in order to enable the device manger A 88 and thecomputer device A to process plural transactions concurrently withtransactions processed by the other device managers 90-92.

Like the device manager A 88, the device manager B 90 includes a queuemanager 120 and a transaction I/O module 122. Similarly, the devicemanager C 92 also includes a queue manager 124 and a transaction I/Omodule 126. The queue managers 120, 124 and the transaction I/O modules122, 126 operate similarly to the queue manager 116 and the transactionI/O module 118 of the device manager A 88 as discussed above.

Based on the foregoing discussion, it will be appreciated that theinvention provides concurrent processing of computer transactions. Inparticular, the transaction queue, transaction controller, and devicemanagers of each bus agent enable each bus agent to process transactionrequests concurrently and still transmit transaction responses acrossthe processor bus according to the sequential order in which thetransaction requests were received. Such concurrent processing enablesthe present invention to process transactions faster than is enabled byprior art non-concurrent transaction processing systems.

It should be understood that even though numerous advantages of thepresent invention have been set forth in the foregoing description, theabove disclosure is illustrative only. Changes may be made in detail andyet remain within the broad principles of the present invention.

We claim:
 1. A method of concurrent transaction processing in a computersystem having one or more computer processors, a bus agent, a computerbus coupling the bus agent to the one or more computer processors, and aplurality of computer devices coupled by the bus agent to the computerbus, the method comprising:transmitting first and second transactionrequests from the one or more computer processors across the computerbus to the bus agent; receiving the second transaction request at thebus agent after receiving the first transaction request at the busagent; transmitting the first transaction request from the bus agent toa first computer device of the plurality of computer devices;transmitting the second transaction request from the bus agent to asecond computer device of the plurality of devices before the bus agenthas received any response to the first transaction request from thefirst computer device; receiving at the bus agent a first transactionresponse from the first device in response to the first transactionrequest; receiving at the bus agent a second transaction response fromthe second device in response to the second transaction request; andtransmitting the first and second transaction responses across thecomputer bus to the one or more computer processors.
 2. The method ofclaim 1, further including:transmitting a third transaction request fromthe one or more computer processors across the computer bus to the busagent; and transmitting the third transaction request to the firstcomputer device from the bus agent before transmitting the firsttransaction response across the computer bus to the one or more computerprocessors.
 3. The method of claim 1 wherein the first transactionrequest is transmitted to the bus agent before the second transactionrequest is transmitted to the bus agent, the first transaction responseis received at the bus agent after the second transaction response isreceived at the bus agent, and the first transaction response istransmitted across the computer bus before the second transactionresponse is transmitted across the computer bus.
 4. The method of claim3, further including:transmitting a third transaction request from theone or more computer processors across the computer bus to the busagent; and transmitting the third transaction request to the secondcomputer device from the bus agent before receiving the firsttransaction response from the first computer device at the bus agent. 5.The method of claim 1, further including:maintaining a transaction queueat the bus agent by inserting the first and second transaction requestsinto the transaction queue according to the order in which the first andsecond transaction requests are received at the bus agent; and using thetransaction queue to ensure that the first and second transactionresponses are transmitted across the computer bus according to the orderin which the first and second transaction requests are received at thebus agent.
 6. The method of claim 5, further comprising:associating afirst device manager of the bus agent with the first computer device,the first device manager employing a first pointer into the transactionqueue to enable the first device manager to keep track of whichtransaction request in the transaction queue is next to be processed bythe first computer device; and associating a second device manager ofthe bus agent with the second computer device, the second device manageremploying a second pointer into the transaction queue to enable thesecond device manager to keep track of which transaction request in thetransaction queue is next to be processed by the second computer device.7. The method of claim 6 wherein in response to receiving the firsttransaction response from the first computer device, the first devicemanager:sets a tag in the transaction queue to indicate that the firsttransaction response in response to the first transaction request hasbeen received; determines whether a subsequent transaction request inthe transaction queue is intended for the first computer device; andupdates the first pointer to point to the subsequent transaction requestif the subsequent transaction request is intended for the first computerdevice.
 8. The method of claim 7 wherein the bus agent includes requestlogic that determines from the tag that the first transaction responseis ready to be transmitted on the computer bus, and in response thereto,transmits the first transaction response on the computer bus.
 9. Themethod of claim 1, further comprising:associating a first device managerof the bus agent with the first computer device, the first devicemanager keeping track of transaction requests intended for the firstcomputer device and transaction responses received from the firstcomputer device; and associating a second device manager of the busagent with the second computer device, the second device manager keepingtrack of transaction requests intended for the second computer deviceand transaction responses received from the second computer device. 10.A method of concurrent transaction processing in a computer systemhaving one or more computer processors, a bus agent, a computer buscoupling the bus agent to the one or more computer processors, and afirst computer device coupled by the bus agent to the computer bus, themethod comprising:transmitting first and second transaction requestsfrom the one or more computer processors across the computer bus to thebus agent; transmitting the first transaction request from the bus agentto the first computer device; receiving at the bus agent from the firstcomputer device a first transaction response in response to the firsttransaction request; and transmitting the second transaction requestfrom the bus agent to the first computer device before the bus agent hastransmitted the first transaction response to the one or more computerprocessors via the computer bus.
 11. The method of claim 10, furtherincluding transmitting a third transaction request from the bus agent toa second computer device coupled to the bus agent after transmitting thefirst transaction request from the bus agent to the first computerdevice and before receiving at the bus agent the first transactionresponse from the first computer device.
 12. The method of claim 11,further including:receiving a second transaction response from the firstcomputer device in response to the second transaction request; receivingat the bus agent a third transaction response from the second computerdevice in response to the third transaction request; maintaining atransaction queue at the bus agent by inserting the first, second andthird transaction requests into the transaction queue, according to theorder in which the first, second, and third transaction requests arereceived at the bus agent; and using the transaction queue to ensurethat the first, second and third transaction responses are transmittedacross the computer bus according to the order in which the first,second, and third transaction requests are received at the bus agent.13. The method of claim 11, further including:associating a first devicemanager of the bus agent with the first computer device, the firstdevice manager keeping track of transaction requests intended for thefirst computer device and transaction responses received from the firstcomputer device; and associating a second device manager of the busagent with the second computer device, the second device manager keepingtrack of transaction requests intended for the second computer deviceand transaction responses received from the second computer device. 14.The method of claim 10, further including storing the first transactionresponse in a storage buffer of the bus agent while transmitting thesecond transaction request to the first computer device.
 15. A method ofconcurrent transaction processing in a computer system having one ormore computer processors, a bus agent, a computer bus coupling the busagent to the one or more computer processors, and a plurality ofcomputer devices coupled by the bus agent to the computer bus, themethod comprising:transmitting first and second transaction requestsfrom the one or more computer processors to the bus agent; transmittingthe first transaction request to a first computer device of theplurality of computer devices after the first and second transactionrequests are received at the bus agent; and transmitting the secondtransaction request to a second computer device of the plurality ofcomputer devices independently from the transmission of the firsttransaction request to the first computer device, the second transactionrequest being transmitted without waiting for the first transactionrequest to be transmitted and the first transaction request beingtransmitted without waiting for the second transaction request to betransmitted.
 16. The method of claim 15, further including:transmittinga third transaction request from the one or more computer processorsacross the computer bus to the bus agent; receiving at the bus agent afirst transaction response from the first computer device in response tothe first transaction request; and transmitting the third transactionrequest to the first computer device from the bus agent beforetransmitting the first transaction response across the computer bus tothe one or more computer processors.
 17. The method of claim 15 furtherincluding:receiving at the bus agent a first transaction response fromthe first computer device in response to the first transaction request;and receiving at the bus agent a second transaction response from thesecond computer device in response to the second transaction request,wherein the first transaction request is transmitted to the bus agentbefore the second transaction request is transmitted to the bus agent,the first transaction response is received at the bus agent after thesecond transaction response is received at the bus agent, and the firsttransaction response is transmitted across the computer bus before thesecond transaction response is transmitted across the computer bus. 18.The method of claim 15, further including:maintaining a transactionqueue at the bus agent by inserting the first and second transactionrequests into the transaction queue according to the order in which thefirst and second transaction requests are received at the bus agent; andusing the transaction queue to ensure that first and second transactionresponses, transmitted in response to the first and second transactionrequests, respectively, are transmitted across the computer busaccording to the order in which the first and second transactionrequests are received at the bus agent.
 19. The method of claim 18,further comprising:associating a first device manager of the bus agentwith the first computer device, the first device manager employing afirst pointer into the transaction queue to enable the first devicemanager to keep track of which transaction request in the transactionqueue is next to be processed by the first computer device; andassociating a second device manager of the bus agent with the secondcomputer device, the second device manager employing a second pointerinto the transaction queue to enable the second device manager to keeptrack of which transaction request in the transaction queue is next tobe processed by the second computer device.
 20. The method of claim 19wherein in response to receiving the first transaction response from thefirst computer device, the first device manager:sets a tag in thetransaction queue to indicate that the first transaction response inresponse to the first transaction request has been received; determineswhether a subsequent transaction request in the transaction queue isintended for the first computer device; and updates the first pointer topoint to the subsequent transaction request if the subsequenttransaction request is intended for the first computer device.